The invention relates to a converter for the conversion of an input voltage between a first supply terminal and a second supply terminal into an output voltage, including switching means which in operation are turned on and turned off alternately under control of a switch control signal, an inductive element which together with the switching means forms a series arrangement coupled between the first supply terminal and the second supply terminal, a control circuit for the supply of the switch control signal, and evaluation means for the evaluation of a voltage across the switching means, which voltage exhibits ringing, and for the supply of an evaluation signal to the control circuit.
FIG. 1 shows such a converter and is known from U.S. Pat. No. 5,754,414. A primary winding of a transformer 12, together with a switching transistor 18, is coupled between a first supply terminal V.sub.S and a second supply terminal GND so as to receive an input voltage V.sub.S. The transformer 12 also has a secondary winding 30. A voltage supplied by the secondary winding 30 is rectified by means of a diode 32 and is subsequently smoothed by a smoothing capacitor 34, which results in an output voltage V.sub.0 between an output terminal V.sub.0 and the second supply terminal GND. The operation of the known converter is now explained with reference to FIGS. 1 to 3. Between the instants to and ti the base voltage V.sub.b (with respect to the second supply terminal GND) of the switching transistor 18 is substantially 0 V, as a result of which the switching transistor 18 is not in conduction. Consequently, the collector V.sub.c (with respect to the second supply terminal GND) of the switching transistor 18 is substantially equal to the input voltage V.sub.S. At the instant t.sub.1 the base voltage V.sub.b is increased to such an extent that the switching transistor 18 is driven into full conduction, as a result of which the collector voltage V.sub.c becomes substantially equal to 0 V. The voltage V.sub.b remains high till the instant t.sub.2. A direct consequence of this is that between the instant t.sub.1 and t.sub.2 the voltage across the primary winding of the transformer 12 is substantially equal to the input voltage V.sub.S, as a result of which energy is stored in the primary winding of the transformer 12. As a result of this, the stored energy is transferred to the secondary winding 30 and eventually, via the rectifier diode 32, to a load (not shown in the Figures), which load may be coupled between the output terminal V.sub.0 and the second supply terminal GND. The abrupt turn-off of the switching transistor 18 at the instant t.sub.2 causes a sudden increase of the collector voltage V.sub.c as a result of the self-inductance of the primary winding. The collector voltage V.sub.C right after the instant t.sub.2 is therefore substantially higher than the input voltage V.sub.S. Subsequently, the collector voltage V.sub.c begins to decrease as indicated by the reference numeral 44 in FIG. 2. From a given instant, indicated by the reference numeral 82, the collector voltage V.sub.C starts ringing. It is to be noted that FIGS. 2 and 3 are shown in said United States Patent Specification in order to illustrate the operation of conventional converters. However, FIGS. 2 and 3 are now also used to illustrate the operation of the converter known from said United States Patent Specification. Thus, said United States Patent Specification states, as shown in FIGS. 2 and 3, that many conventional converters operate with a fixed switching frequency f. The switching frequency f is then low enough to achieve that at an instant t.sub.3, which corresponds to the instant t.sub.1, the collector voltage V.sub.C exhibits hardly or no ringing, as a result of which at the instant t.sub.3 the switching transistor 18 can be turned on again without the risk of the collector voltage V.sub.c being much higher than the input voltage V.sub.S, as a result of which the switching transistor 18 could be damaged. Moreover, at the instant that the collector voltage V.sub.C has stopped ringing it can be ascertained that the energy transfer from the transformer 12 to the load has ceased. Since such conventional converters operate with a fixed switching frequency f which is so low that it is certain that at the instant t.sub.3 the collector voltage V.sub.C has stopped ringing, such conventional converters are not suitable for uses which require a high switching frequency f. For this reason, steps have been taken in the known converter as disclosed in said United States Patent Specification, to render the converter suitable for operation with a high switching frequency f The essence is that the converter each time, i.e. also when the ringing frequency changes, adapts the switching frequency f in such a way that the switching transistor 18 is turned on at the first minimum of the ringing collector voltage V.sub.C, which first minimum bears the reference numeral 84 in FIG. 2. The energy transfer from the transformer 12 to the load has then not yet been completed. However, when it is born in mind that ringing does not arise until the instant at which the rectifier diode 32 is leaving the conductive state, it is evident that the major part of the energy has already been transferred to the load. The ringing frequency of the collector voltage V.sub.C depends, for example, on the type of transformer used. For this reason, said United States Patent Specification states that the switching frequency f of the converter should be variable and should automatically adapt itself so as to achieve that the instant t.sub.3, as shown in FIG. 2, substantially coincides with the first minimum 84 of the ringing collector voltage V.sub.C. Said United States Patent Specification therefore explicitly states that the conventional converters having a fixed switching frequency are unfavorable. This is because the switching transistor 18 then cannot always be turned on at the first minimum 84 of the ringing collector voltage V.sub.C.
A disadvantage of the known converter as described in said United States Patent Specification is that it is not suitable for uses where a substantially constant switching frequency is desirable. This may, for example, result in the switching frequency assuming a very high value, which may adversely affect the efficiency of the converter.